This invention relates to rangefinders for optical apparatus such as a camera and, more particularly, to rangefinders which measure the distance to an object by means of a pair of optical sensor arrays and a combination of electronic circuits.
Rangefinders using optical sensor arrays have been known for a long time and recently completely electronic rangefinders without any moving parts have appeared. Such electronic rangefinders are highly regarded because they are compact, inexpensive and accurate. However, the presently available electronic rangefinders require too much time for most rangefinding purposes and therefore they have not been widely used.
A diagram illustrative of the principle of the operation of a conventional rangefinder is shown in FIG. 1. In that diagram, light from an object 1 is incident on two small lenses 2 and 3 which have a sufficiently short focal length f that light rays received from the object through different spaced paths 4 and 5 produce corresponding spaced images 7 and 8 in a focal plane 6 which is common to the lenses 2 and 3. When the object 1 is at an infinite distance, the centers of the images 7 and 8 are located at reference positions 70 and 80 in FIG. 1, but when the object 1 is located at a closer distance, the centers of the images are shifted apart to positions 71 and 81. If the distance by which the images 7 and 8 are shifted from the reference positions 70 and 80 are designated x.sub.1 and x.sub.2, respectively, then the total shift x may be expressed as follows: EQU x=x.sub.1 +x.sub.2 =b.multidot.f/d (1)
Thus, the distance d to the object 1 can be measured by d=b.multidot.f/x. In this case, b is the distance between the optical axes of the small lenses, that is, the base length.
To obtain the shifted amounts x.sub.1 and x.sub.2, or the sum x of both, two optical sensor arrays 10 and 11 are provided in the focal plane 6 as shown in FIG. 1. These optical sensor arrays each comprise a plurality of optical sensors, for instance CCD devices, and an analog photoelectric signal is generated by each optical sensor corresponding to the light intensity at the portion of the image which is incident on the sensor. Each analog signal is converted into a "0" or "1" digital signal by a device for quantifying the analog signal, for instance, an analog/digital converter. FIG. 2 shows a typical circuit for obtaining the sum x of the shifted distances by comparing two image signal trains comprising the digital image signals from the left and right optical sensor arrays. The circuit includes two shift registers 12 and 13 and the output signals from the optical sensor arrays 10 and 11 of FIG. 1 are quantified into digital image signals by the analog/digital converter and stored in these shift registers in the same order as the arrangement of the optical sensors in the optical sensor arrays.
When the above-described image signals are written into the shift registers 12 and 13, shift signals are applied from a timing control unit 14 to the control terminals of the shift registers 12 and 13 and the image signals that have been stored in each of the shift registers 12 and 13 are shifted one stage at a time in synchronism and successively produced at the shift register output terminals OUT. The output signals from the shift registers 12 and 13 are respectively returned to the corresponding input terminals IN and again entered in sequence into the shift registers. An exclusive NOR circuit 15 generates a "1" when the outputs of the shift registers 12 and 13 coincide with each other and a "0" when the outputs do not coincide. A coincidence counter 16 counts the number of times the output of the exclusive NOR circuit is "1" during the time when the image data signals are produced successively by the shift registers 12 and 13.
Assuming that the number of optical sensors in the optical sensor arrays 10 and 11 shown in FIG. 1 are m and n, respectively, and that m and n pieces of image data are stored in the shift registers 12 and 13 with m&lt;n, if the signals are shifted m times by the shift registers 12 and 13 there is a comparison of all of the data stored in the shift register 12 with the first m pieces of data stored in the shift register 13. During this comparison, the counter 16 counts the number of bits which coincide when the image data stored in the shift registers 12 and 13 have not been shifted with respect to each other. When this comparison has been completed, the signals in the shift register 12 have been returned to the initial positions, whereas the signals in the shift register 13 have been shifted m positions to the right. At this point, the contents of the coincidence counter 16 are stored in a maximum coincidence number memory 17. Then the signals in the shift register 13 are shifted by (n-m +1) positions under the command of the timing control unit 14 and the counter 16 is cleared. As a result, the contents of the shift register 13 are located one position to the right as compared with their initial condition. A shift counter 18, which counts the number of positions the signals in the register 13 have been shifted to the right as compared with the initial condition, is incremented each time the comparison of data in the shift registers has been completed.
The second comparison is made by shifting the data in the shift registers 12 and 13 successively to the right m times. When the second comparison has been completed, the contents C.sub.1 of the counter 16 and the contents C.sub.2 of the maximum coincidence number memory 17 are compared by a comparator 19 to determine which is larger. If C.sub.1 &gt;C.sub.2, C.sub.1 is transferred to the maximum coincidence number memory 17. At the same time, the contents S.sub.1 of the shift counter 18 are written to a shift memory 20. Then the shift register 13 is again shifted by (n-m+1) positions to the right as in the preceding case and the counter 16 is cleared.
This is followed by the comparisons of the stored contents of the shift registers 12 and 13 and of contents of the counter 16 and the maximum coincidence number memory 17; rewriting of the maximum coincidence number memory 17 and the shift memory 20 based on the results obtained; shifting of the shift register 13 (n-m+1) times; and clearance of the counter 16. When the repetition is completed, the number stored in the maximum coincidence number memory 17 is the maximum coincidence number resulted from successive examination of the degree of coincidence between the contents of the shift register 12 and part of the contents of the shift register 13. In the shift memory 20, on the other hand, the shift number between the shift registers 12 and 13 which provides the maximum coincidence corresponding to the difference in the signal positions is stored. This shift number corresponds to the total image shift x. The timing control unit 18 then latches the contents of the shift memory 20 in an output latch as the final step of the operation and causes the latch to produce a range signal.
In the above-described prior art comparator for comparing image data in a rangefinder, the disadvantage is that a long time is unavoidably required to compare image data because the maximum coincidence point between two image data trains must be determined by repeatedly circulating the contents of the shift registers 12 and 13 while changing the relative shift number.
Since such a long time is needed to compare image data and thus to determine the distance to an object, when photographs are taken with a video camera, for instance, while the visual fields are moving, the images that are photographed may be out of focus since the rangefinder has focused on visual fields other than the one being taken. Moreover, such rangefinders require circuits for determining the distance d from the quantity x according to the equation (1) and for bringing an object into focus based on the quantity x, as well as circuits suited to the characteristics of optical equipment for controlling and regulating other parts of the optical equipment in which the rangefinder has been incorporated, for instance, diaphragms and shutters in the case of cameras. It is, needless to say, most desirable to integrate a rangefinder with computing, control and regulating circuits, or build them into a custom integrated circuit. However, because the functions required for such integrated circuits will vary according to the specific optical equipment, it is difficult to design an integrated circuit which can deal with each of such requirements and such an effort would also be uneconomical. On the other hand, because preparation of software that meets such functional requirements will be easy if a microcomputer is used, this choice is obviously advantageous. However, the short time permitted for rangefinding as a practical matter makes such a choice inappropriate.
Accordingly, it is an object of the present invention to eliminate the shortcomings and incompatibilities inherent in the prior art rangefinders and to provide a rangefinder having a short rangefinding time which is capable of readily meeting requirements peculiar to different applications.